Soldering is widely used in electronics fabrication for interconnecting components. This may be machine soldering where substantial numbers of components are involved or many substantially similar products are produced so as to make automated soldering economical. There is still, however, a large amount of hand soldering of components. In such work a soldering iron is hand guided to the joint.
Whether machine soldering or hand soldering, it is often desirable to monitor the temperature of the joint or soldering iron. Too low a temperature may result in "cold" joints which do not provide long term reliability. Too high a temperature may result in damage to sensitive electrical components. To maintain an appropriate range of temperatures, it may be desirable to monitor the temperature of the tip of a soldering iron. Based on such feedback information, the temperature can be manually or automatically controlled.
Temperature of a soldering iron tip may be measured by inserting or attaching a thermocouple to the tip. This technique is to measure infrared emittance from the surface of the tip. There have been shortcomings in this technique since the emittance from the surface depends on important properties of the surface in addition to temperature. For example, the emissivity of the soldering iron tip is apt to change as the metal oxidizes. Tips are not necessarily uniform in surface emissivity due to variations in the manufacturing process. The surfaces are often plated with reflective metals that have low emissivity and high reflectance, making accurate measurements rather difficult and unreliable.
It is, therefore, highly desirable to provide a soldering iron tip having high and uniform emissivity and one that retains uniform emissivity during use.
High emissivity paints are sometimes applied to soldering iron tips, however, this affords a temporary solution. Such paint tends to flake off the surface after a period of use. It is also subject to easy removal during the frequent cleaning of soldering iron tips. Removal of excess flux from a tip is often by use of a wet sponge. The thermal shock of this cleaning is sometimes too much for brittle, relatively low adhesion paint coatings.
It is also desirable to keep molten solder from travelling too far up the tip. Many soldering iron tips are chromium plated in a region beyond the end since solder does not wet the chromium and can be kept confined to the end of the tip. Chromium plating is undesirable because of health and safety concerns in the plating shop. A low cost substitute for chromium plating is desirable.
Whether machine soldering or hand soldering, it is also desirable to prevent electrical leakage from reaching the soldering iron tip. This may be required when soldering sensitive electronic components. For example, USS. Department of Defense specification DOD-STD-2000-1A states "protective devices to prevent potential differences greater than 2 millivolts (mV) shall be used when voltage sensitive devices are being soldered."
Only the soldering iron tip contacts the leads to electronic components. Thus, electrical isolation of the tip from the balance of the soldering iron is desirable.
It would be particularly desirable to have a way of electrically isolating a soldering iron tip, controlling wetting of the surface by molten solder, and providing high and uniform resistivity, all with a single and inexpensive treatment.